Lumber drying process



F. D. ZO T'TU LUMBER DRYING PROCESS Feb. 24; 1970 Filed July 12, 1968 2Sheets-Sheet l Feb. 24, 1970 P; D. ZOTTU LUMBER DRYING PROCESS 2Sheets-Sheet 2 Filed July 12, 1968 United States Patent 3,496,645 LUMBERDRYING PROCESS Paul D. Zottu, 30 Washburn Ave., Needham, Mass. 02192Filed July 12, 1968, Ser. No. 744,384 Int. Cl. F26b 3/34 US. Cl. 34-1 6Claims ABSTRACT OF THE DISCLOSURE A process of drying lumber by highfrequency heating combined with controlled atmosphere drying. Highfrequency heating is applied to maintain the interior of the lumber at atemperature higher than that of the surface. The heat required forsurface evaporation is supplied by a circulating controlled atmospherewhich is maintained at a temperature higher than the free boiling pointof the moisture at the lumber surface throughout the drying cycle.

BACKGROUND OF THE INVENTION This invention relates to lumber drying andmore particularly to processes of drying lumber by means of a highfrequency electric field in combination with a circulating atmospherecontrolled with respect to temperature and humidity.

One of the customary methods of drying lumber is kiln drying bycirculation of a heated atmosphere of controlled humidity. In thisprocess the heat required for evaporation is applied at the surface ofthe Wood, and the temperature in the interior of the wood is lower thanthat nearthe surface. This temperature gradient creates a moisture vaporpressure gradient which opposes movement of moisture from the interiorof the wood to the surface.

Heating by means of high frequency electric field appears to offer someadvantages in drying lumber, because the wood is heated internally,reversing the normal temperature, moisture and vapor pressure gradientsand causing more rapid migration of the moisture from the interior tothe surface. If high frequency energy is used to supply all the heatnecessary for surface evaporation, the process is quite expensive.Drying by high frequency energy alone has been found technically andeconomically feasible only for short pieces of expensive wood.

Experiments have also been made with drying of stacks of lumber usinghigh frequency energy in combination with a circulating atmosphereheated by a less expensive energy source, such as steam, to atemperature of 212 degrees F. In this case, part of the heat requiredfor evaporation is supplied by the atmosphere. This appears acceptablefor drying green lumber to the fiber saturation point, about 30%moisture content. Below this point the bound water contained in the woodfiber boils at temperatures higher than 212 degrees F. and in anatmosphere of 212 degrees F., the additional heat required forevaporation must be supplied by the high frequency source, so that theprocess becomes unduly expensive.

The principal object of this invention is to provide a method of dryinglumber in commercial quantities, either.

from green state or from air dried state, to any desired moisturecontent below the fiber saturation point, more quickly than is possibleby methods previously used, without damage to the lumber and at a costcompetitive with present kiln drying processes. Other objects,advantages,

and novel features will be apparent from the following description.

SUMMARY The process here disclosed combines the use of high frequencyheating and circulating atmosphere heating. High frequency energy isrelied on to maintain a temice perature gradient favorable to migrationof water toward the surface of the lumber and the heat required forsurface evaporation is supplied by the circulating atmosphere.

Throughout the drying process the atmosphere is maintained at atemperature at or above the free boiling point of the moisture at thelumber surface. At a moisture content above the fiber saturation point,about 30%, the excess moisture is essentially free water and the freeboiling point is 212 degrees F. Below the fiber saturation point themoisture in the wood is a water solution of sap constituents and itsfree boiling point rises as it becomes more concentrated. At 8% moisturecontent, the free boiling point is about 245 degrees F. and at 1 2%moisture content the free boiling point is around 300 degrees F. Thetemperature of the kiln atmosphere is raised progressively as themoisture content of the lumber is reduced from the fiber saturationpoint to the final content desired.

According to one manner of performing the process, boards are stacked inthe usual manner in a stationary kiln which has electrodes on eitherside of the stack. A high frequency energy is applied to the electrodescontinuously and the conditioned atmosphere is circulated around thelumber. The process may also be performed in a continuous kiln in whichstacks of lumber are moved on cars through high frequency fields appliedby a series of pairs of electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings illustrating theinvention:

FIGURE 1 is a vertical cross-section through a kiln suitable for dryinga stationary stack of lumber according to the process of the invention;

FIGURE 2 is a cross-section of reduced scale, partly broken away, takenalong line 22 of FIGURE 1;

FIGURE 3 is a vertical cross-section similar to FIG- URE 1 showing analternative arrangement of electrodes;

FIGURE 4 is a cross-section of reduced scale taken along line 44 ofFIGURE 3; and

FIGURE 5 is a vertical cross-section illustrating a continuous type ofkiln for performing the process.

DETAILED DESCRIPTION FIGURES 1 and 2 illustrate, somewhat schematicallya typical closed kiln 10 for performing the drying process. A stack ofboards 12, stacked with spacers 11 in the usual manner, is carried on aflat car 13. The stack is placed between a pair of electrodes 14. Asbest shown in FIG. 2, each electrode is a rectangular plate, .largeenough to cover the entire side of the lumber stack, provided withperforations 14a to allow circulation of the atmosphere. A coarse metalmesh would also be suitable for the electrodes. The electrodes areconnected to the terminals of a coaxial cable 15 which is connected to asuitable source (not shown) of high frequency electric energy, of afrequency in the neighborhood of 1 to megacycles.

The kiln atmosphere is heated by steam or electric coils 16, andcirculated by a fan 17. The steam may be generated by the mosteconomical fuel available at the particular location. Means are providedto regulate the moisture of the circulating atmosphere. The humidity maybe raised by introducing a water spray, or lowered by venting andintroduction of new air. The process is performed as follows:

GREEN LUMBER To dry lumber from green to fully dried state, theatmosphere in the kiln is heated to 212 degrees F. or above and the highfrequency field is applied to the stack to produce a surface lumbertemperature somewhat below that of the atmosphere. The relative humidityis main- 3 tained at a high value. These conditions are maintained untilthe moisture content in the wood is reduced to the fiber saturationpoint.

The free boiling point of the moisture in the wood increases withdecreasing moisture content, from the fiber saturation point down. Atthe fiber saturation point the free boiling point is 212 degrees. At 8%the free boiling point is about 245 degrees, and at 1% about 300degrees. As drying progresses below the fiber saturation point, thetemperature of the kiln atmosphere is progressively raised, and alwaysmaintained at or above the free boiling point. The high frequencyheating is controlled to keep the temperature or" the wood surface belowthat of the atmosphere.

AIR DRIED LUMBER This process is especially advantageous for drying airdried lumber, which normally has a moisture content of 25% to 30%. Todry air dried lumber, the kiln atmosphere is heated to, or above, thefree boiling point of the moisture in the lumber, 212 degrees F. in thecase of lumber of 30% moisture content, about 220 degrees F. in the caseof lumber of 25% moisture content. The relative humidity of theatmosphere is maintained at a high value. The high frequency heating isapplied to heat the surface of the lumber to a temperature below that ofthe atmosphere. The temperature of the atmosphere is raised, as themoisture content is reduced, to keep the temperature always at, orabove, the free boiling point. The temperature of the wood surface ismaintained by the high frequency fieid at a temperature somewhat belowthat of the atmosphere.

FIGURES 3 and 4 disclose an alternative arrangement of the electrodesfor supplying the high frequency energy tothe stack. One electrode 18 isplaced horizontally in the center of the stack, and two otherelectrodes, 19 and 20 are placed horizontally across the top and bottomof the stack respec 'vely. The electrode 18 is connected to one terminalof the cable 15, and electrodes 19 and 20 are connected to the otherterminal to set up fields of opposite polarity in the top and bottom ofthe stack. The electrodes may have perforations, as exemplified by theperforations 21 in FIGURE 4, or can be made of a continuous sheet. Thedrying process is performed in the same manner as with the electrodearrangement shown in FIGURE 1.

This method of placing electrodes may be extended to any number ofelectrodes but preferably an odd number. Every other electrode isconnected to one lead of the power source and the remaining electrodesare connee-ted to the other lead. If one of the power leads is at groundpotential it is better to connect the set with the two outsideelectrodes, of a stack with an odd number, to the ground lead.

FIGURE illustrates a continuous type of kiln 22. Stacks of iumber 23 aremounted on cars 24 which run on rails 25 through the kiin. The carsenter at the left hand end of the kiln and travel in the direction ofarrow 26. Pairs of electrodes 27 are mounted at intervals on either sideof the track so that the cars pass between them. The drying process iscarried out essentially the same way as in a stationary kiln, that isthe atmosphere is maintained at a temperature at, or above, the freeboiling point of the moisture in the wood. In order to accomplish this,it is necessary to raise the temperature progressively toward the exitend of the kiln. The electrodes provide high frequency heating tomaintain the desired temperature gradient in the wood.

Means for controlling the temperature and humidity of the kilnatmosphere and the output of the high frequency sources to maintain thedesired internal temperature in the wood are not shown, as these areweli known.

4 GENERAL CONSIDERATIONS AND ADVANTAGES OF THE PROCESS Lumber drying bymeans of a controlled atmosphere alone is relatively slow becausemoisture can be removed only as fast as it reaches the surface. Thetemperature gradient set up by heating the lumber from the outside in isinherently adverse to rapid migration of water from the interior to thesurface.

The use of high frequency heating alone is excessively costly because ofthe substantial amount of heat energy required for evaporation.

The principal requirements in practicing the process here disclosed arethat the temperature of the atmosphere be maintained at or above thefree boiling point of the moisture on the surface of the wood, and thatthe temperature of the interior of the wood be maintained higher thanthat of the surface. Under these conditions, high frequency heating isused only to create a temperature gradient more favorable to rapidmigration of moisture to the surface than the gradient which exists whencontrolled atmosphere drying is used alone. This can be done withrelatively low power consumptions. All the heat required for evaporationis supplied by the atmosphere which can be heated by a less expensivesource of energy.

By use of the process, lumber can be dried much more rapidly than by acontrolled atmosphere alone, and at comparable cost. The overall cost ofoperation can be considerably reduced, because of more rapid turnoverand the reduction in inventory needs.

What is claimed is:

1. A process of drying lumber to a moisture content below the fibersaturation point which comprises simultaneously heating the lumber by ahigh frequency field to an internal temperature at least equal to theboiling point of moisture contained internally in the lumber, thereby todrive the moisture to the surface portion of the lumber forrecondensation therein, and circulating a heated atmosphere around thelumber, the atmosphere being continuously maintained at a temperature atleast as high as the free boiling point of condensed sap and moisture inthe surface portion of the lumber throughout the drying process, thetemperature in the surface portion of the lumber being maintained belowthe temperature of the atmosphere throughout the drying process.

2. A process as described in claim 1, in which the temperature of theatmosphere is raised in correspondence to the rise in the free boilingpoint of the sap and moisture at the surface portion of the lumber, asthe moisture content of the lumber is reduced from the fiber saturationpoint to the final moisture content.

3. A process as described in claim 1, in which the lumber is arranged ina stack and disposed between a pair of electrodes which supply the highfrequency field.

4. A process as described in claim 1, in which the lumher is arranged ina stack with one electrode in the center and electrodes on the top andbottom for supplying the high frequency fieid.

5. A process as described in claim 1, in which the lumber is passedbetween successive pairs of electrodes which supply the high frequencyfield.

6. A process as described in claim 1, in which the lumber is arranged ina stack and disposed between a plurality of electrodes, adjacentelectrodes being of opposite polarity.

References Cited UNITED STATES PATENTS 2,567,983 9/1951 Wood 34--l3,031,767 5/1962 Wood -1 341 3,403,450 10/1968 Mann et a1 341 WILLIAM E.WAYNER, Primary Examiner

